Polyethylene terephthalate (PET) has many valuable characteristics including toughness, solvent resistance, high-gloss, and the like. Articles may be fabricated from PET by a number of well known techniques including injection molding, extrusion and the like depending on the shape of the desired product. Certain techniques, in particular, blow molding and extrusion, require the molten polyethylene terephthalate have a suitably high melt viscosity, in excess of 10,000 poises, to prevent collapse or blow outs in the molten preformed state. It is well known that high melt viscosities can be achieved much easier if small amounts of a tri or higher functional ester-forming branching component is included in the polyester. Furthermore, it is still easier if the branched copolyester is subjected to solid state polymerization.
It has now been discovered that branched polyethylene terephthalate of high melt viscosity can be obtained if a small amount of isophthalic acid is intimately blended with the branched copolyester before solid stating. Consequently, the polymer melts are free from inhomogeneities.
U.S. Pat. No. 4,147,738 issued to General Electric discloses a process by which high melt viscosity polyesters are prepared by a two-step process involving incorporating a branching agent in a melt phase polyester and polymerizing to a melt viscosity less than 3,000 poises. Next, blending the melt phase polyester with up to 5 weight percent or an aromatic polycarbonate by extrusion, and solid stating the blend to a melt viscosity suitable for extrusion blow molding, usually greater than 10,000 poises.
Another patent (U.S. Pat. No. 4,132,707) discloses preparing a linear (no-branching agent) polyester precursor having a relatively low melt viscosity which can be handled on conventional polyester melt phase equipment. The melt phase precursor is blended with up to 3 mole percent branching agent by melt blending. The blend polyester-branching agent is further reacted to reach the desired, usually greater than 10,000 poises, melt strength suitable for the required end use.